Vinyl chloride resins (hereinafter referred to as PVC) are easily processed thermoplastic resins since they are easily softened at 150 to 170° C. Vinyl chloride resins have superior water resistance, chemical resistance and petroleum resistance, and are hard, whereby they are used in lining plates, pumps, tanks, plating water baths, treatment water baths, and the like. Since vinyl chloride resins are softened due to addition of a large amount of plasticizer (40 to 80%), they are used in sheets, sides, films, tiles and the like.
PVC manufacturing methods using bulk polymerization do not use a heating medium such as water unlike suspension polymerization and emulsion polymerization, and polymerization thereof is performed using only a vinyl chloride monomer initiator and, as needed, a reaction additive. After polymerization, PVC may be obtained without a drying process.
In addition, in bulk polymerization, an apparatus for bulk polymerization is simple, reaction rate is fast, yield is high, high-purity polymers may be obtained, and polymers may be intactly handled. However, there are drawbacks such as difficult temperature control to strong heating of polymerization systems, extended molecular weight distribution of polymers and difficult precipitation. Therefore, since a heating medium for heat removal is not present in bulk polymerization, it is very important to maintain heat removal stability.
A vinyl chloride monomer and initiator are only supplied into a reactor for bulk polymerization before polymerization, and water as a heating medium, as in emulsion polymerization or suspension polymerization, is not supplied. Accordingly, in preparing PVC using bulk polymerization, while the amounts of liquid-phase vinyl chloride monomers in a reaction system decrease and the amount of solid-phase PVC increases, as polymerization proceeds, solid-phase PVC is distributed in liquid-phase vinyl chloride monomers. In particular, since the specific gravity of PVC (1.35 to 1.45) is greater than that of the vinyl chloride monomer (approximately 0.97), PVC is mainly present in an upper portion of a reactor.
As polymerization progresses, the amounts of vinyl chloride monomers decrease and the amount of PVC increase. Accordingly, all PVC generated in an initial reaction step may maintain a state of being surrounded with vinyl chloride monomers, but, accordingly polymerization progress, the amount of PVC increases and the amounts of vinyl chloride monomers decrease, whereby it become impossible to maintain the state that solid-phase PVC is surrounded with liquids, namely, liquid-phase vinyl chloride monomer. Accordingly, liquidity of PVC decreases and an average distance between PVC particles is shortened, whereby fine particles may be generated due to excessive agglomeration among particles or friction among particles.
In addition, vaporization of liquid-phase vinyl chloride monomers is induced by properly removing gas-phase vinyl chloride monomers in a reaction system, when liquid-phase vinyl chloride monomers are sufficiently present, whereby it is possible to stably maintain proper reaction temperature (namely, polymerization temperature) by removing reaction heat (namely, polymerization heat). However, it is difficult to remove reaction heat when liquid-phase vinyl chloride monomers are deficient, whereby possibility of a local heating phenomenon increases. Accordingly, an agglomeration phenomenon due to fusion among PVC particles may be induced.
The problems are directly related to problems such as abnormal products (fine particle products, mass products due to agglomeration and the like), PVC production having non-uniform particles and the like.
Until today, technical development has focused on collecting fine particles by additionally installing a particle separator after reaction, in order to reduce the amounts of fine particles. Here, when a screening method generally used to remove fine particles is used, a screener is easily blocked due to a static feature of bulk polymerization particles. To complement this, U.S. Pat. No. 4,963,634 introduced a collection method using air movement. However, since this technology is costly and it is not suitable for using collected fine particles, the method is not a fundamental solution. In addition, although an antistatic agent is added to easily isolate fine particles by preventing static between PVC particles, generation of fine particles and scale may not be prevented by the antistatic agent added after polymerization.
There is a need for a method to fundamentally prevent change of normal particles into fine particles due to strong external force applied to particles during conventional bulk PVC polymerization.